1. Field of the Invention
The present invention relates to a separator for a fuel cell which is mainly used as a cell for an electric vehicle, and also to a method of producing the separator, and more particularly to a separator for a fuel cell of the electrolyte type or the phosphoric acid type, and also to a method of producing the separator. In a fuel cell of such a type, a unit cell which is a unit constituting the cell is configured by: sandwiching a gas diffusion electrode having a sandwich structure wherein an electrolyte membrane is configured by an ion exchange membrane, between an anode and a cathode; sandwiching the gas diffusion electrode between separators; and forming fuel gas passages and oxidant gas passages between the separators, and the anode and the cathode.
2. Description of the Prior Art
In a fuel cell, a fuel gas containing hydrogen is supplied to an anode, and an oxidant gas containing oxygen is supplied to a cathode, so that, in the anode and the cathode, electrochemical reactions indicated by the formulae:
xe2x80x83H2xe2x86x922Hxe2x88x92+2exe2x88x92xe2x80x83xe2x80x83(1)
(xc2xd)O2+2Hxe2x88x92+2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
occur, and, in the whole of the cell, an electrochemical reaction indicated by the formula:
H2+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
proceeds. The chemical energy of the fuel is directly converted into electrical energy, with the result that the cell can exert a predetermined performance.
A separator for a fuel cell of the electrolyte type or the phosphoric acid type in which such energy conversion is conducted is desirably gas-impermeable, and also is made of an electrically conductive material. Conventionally, it is known that, as a material meeting the requirements, an electrically conductive resin is used. An electrically conductive resin is a complex which is configured by bonding graphite (carbon) powder by means of a thermosetting resin such as phenol resin, or a so-called bondcarbon (resin-bonded carbon) compound. A separator for a fuel cell is configured by forming such a bondcarbon compound into a predetermined shape.
Conventionally, a separator for a fuel cell having a predetermined shape is formed by using such a bondcarbon compound in the following manner. With respect to the composition ratio of a thermosetting resin such as phenol resin and graphite powder, 25 to 60 wt. % of the thermosetting resin is used as an adequate content in consideration of fluidity, moldability, and gas-impermeability of the bondcarbon compound, and in order to ensure the strength (compression and bending) sufficient for preventing the separator from suffering damage such as a breakage due to vibrations or the like which may be produced during a handling operation in an assembling step of a unit cell of a fuel cell, or a use in an automobile.
In a conventional separator for a fuel cell which is configured by using a bondcarbon compound of such composition ratios, the content of a thermosetting resin serving as an electrically insulating material is large, and hence the conductivity of the separator itself is lowered so that the electrical resistance is increased. This is not preferable from the viewpoint of the performance of a fuel cell.
In order to improve the conductivity of a separator for a fuel cell s configured by using a bondcarbon compound, it has been contemplated that the content of a thermosetting resin be reduced as far as possible. When the content of a thermosetting resin is reduced, however, elongation and fluidity of the bondcarbon compound during a molding process are lowered to impair moldability, and the strength is low. When the resin content is 10 wt. % or less, particularly the strength of a separator becomes insufficient, and therefor the separator easily suffers damage such as a breakage or a crack due to vibrations or the like which are continuously applied to the separator in the case where the separator is used in an automobile.
By contrast, in the case where the resin content is set to the above-mentioned adequate range (25 to 60 wt. %), elongation and fluidity of a bondcarbon compound are excellent and moldability is higher, and strength sufficient for preventing a separator from suffering damage such as a breakage or a crack due to vibrations or the like can be ensured. However, the contact resistance with respect to an electrode and serving as the primary factor which largely affects the performance of a fuel cell becomes higher, as the resin content is larger. When the resin content is larger than 40 wt. %, particularly, the contact resistance is suddenly increased, and the performance of the fuel cell is extremely lowered.
The contact resistance serving as the primary factor which largely affects the performance of a fuel cell will be considered. Even when a fuel cell is used in an automobile in which vibrations are always applied to the fuel cell, it is desirable that the contact resistance be stably maintained to 10 mxcexa9xc2x7cm2 or lower. When the contact resistance is to be stably maintained to such a requested value, a countermeasure in which only the composition ratios of a thermosetting resin and graphite powder are considered cannot satisfy both the requirements on fluidity and moldability of a compound and the strength of a molded member (separator), and the contact resistance, as described above. Development of a separator for a fuel cell which is excellent in moldability and strength, and which can be stably maintained to a low contact resistance of 10 mxcexa9xc2x7cm2 or lower is strongly requested. At present, however, there exists no separator which can satisfy the noted desirability.
The present invention has been conducted in order to satisfy the noted desirability. It is an object of the invention to provide a separator for a fuel cell which is excellent in fluidity and moldability, and in which, while ensuring strength sufficient for preventing the separator from suffering damage such as a breakage due to vibrations or the like, the contact resistance can be set to a value lower than a requested value, and the low contact resistance can be stably maintained.
It is another object of the invention to provide a method of producing a separator for a fuel cell wherein, even when a molding material of low fluidity is used, a separator which has a uniform and correct shape, and in which a low contact resistance can be stably maintained can be surely produced.
In order to attain the objects, the separator for a fuel cell of the invention is a separator for a fuel cell consisting of a complex which is configured by bonding graphite powder by means of a thermosetting resin, and characterized in that, in the complex, a composition ratio of the graphite powder is set to 60 to 90 wt. %, a composition ratio of the thermosetting resin is set to 10 to 40 wt. %, and an average particle diameter of the graphite powder is set to a range of 15 to 125 xcexcm.
In the complex, preferably, the composition ratio of the graphite powder is set to 70 to 87 wt. %, and the composition ratio of the thermosetting resin is set to 13 to 30 wt. %. Preferably, the average particle diameter of the graphite powder is set to a range of 40 to 100 xcexcm.
In order to meet the above-mentioned demands for development, intensive studies on a separator for a fuel cell which is configured by using a bondcarbon compound have been conducted, and finally found that the contact resistance serving as the primary factor which largely affects the performance of a fuel cell is determined not only by the composition ratios of a resin and graphite powder, the average diameter of the graphite powder closely affects the performance at the highest degree, the contact resistance is largely varied depending on the size of the average diameter, and the average diameter of the graphite powder is closely related also to fluidity, moldability, and strength of the compound. Based on this finding, the composition ratios of a resin and graphite powder, and the average diameter of the graphite powder have been respectively set to the above-mentioned ranges, thereby completing the invention.
According to the thus configured invention, as the graphite powder which is the one composition of the complex and which affects the contact resistance at the highest degree, graphite powder in which the average diameter is set to a range of 15 to 125 xcexcm, preferably, 40 to 100 xcexcm is used, the composition ratio of the thermosetting resin which is the other composition of the complex, and which largely affects fluidity, moldability and strength is set to a range of 10 to 40 wt. %, preferably, 13 to 30 wt. %, thereby attaining an effect that, while the complex serving as a molding material has excellent elongation and fluidity and exerts high moldability, and strength sufficient for preventing the separator from suffering damage such as a breakage or a crack due to vibrations or the like can be ensured, the contact resistance with respect to an electrode can be set to a low value of 10 mxcexa9xc2x7cm2 or lower which is required in a separator for a fuel cell, and the low contact resistance can be stably maintained so that the performance of a fuel cell can be remarkably improved.
In the case where the average particle diameter of graphite powder is smaller than the above-mentioned range, or, for example, 10 xcexcm or smaller, the contact resistance is higher or 15 mxcexa9xc2x7cm2 or more, even when the resin content is adjusted to any value. Namely, the obtained contact resistance is very different from the value (10 mxcexa9xc2x7cm2 or lower) which is required in a fuel cell to be used under conditions where vibrations are applied, such as the case of mounting on an automobile. In the case where the resin content is smaller than 10 wt. %, and also in the case where the average diameter of graphite powder is, for example, 150 xcexcm or more, i.e., exceeds the above-mentioned range, fluidity and moldability are improved, but a large number of breakages, minute cracks, and the like are produced by vibrations in edges of projections serving as contact faces with respect to an electrode. Even when a low contact resistance is obtained in an early stage of use, the contact resistance is suddenly increased after use of a short time, so that a low contact resistance meeting the above-mentioned demands cannot be maintained. This will be described later in detail.
In the separator for a fuel cell of the invention, when a surface roughness of a portion contacting an electrode is set to a range of Ra=0.1 to 0.5 xcexcm as measured by a surface roughness meter having a probe of a diameter of 5 xcexcm, the contact resistance can be further lowered, so that further improvement of the performance of a cell can be attained.
The method of producing a separator for a fuel cell according to the invention is a method of producing a separator for a fuel cell configured by molding a complex in which composition ratios are set to 60 to 90 wt. % of graphite powder, and 10 to 40 wt. % of a thermosetting resin, and an average diameter of the graphite powder is set to a range of 15 to 125 xcexcm, and characterized in that the complex is previously coldmolded into a shape similar to a final molded shape by a pressure of a range of 2 to 10 MPa, the preliminary molded member is then placed in a mold, and the preliminary molded member is molded into the final shape by applying a pressure of a range of 10 to 100 MPa.
Preferably, in the complex, the composition ratio of the graphite powder is set to 70 to 87 wt. %, the composition ratio of the thermosetting resin is set to 13 to 30 wt. %, and the average particle diameter of the graphite powder is set to a range of 40 to 100 xcexcm.
The shape similar to a final molded shape means that the dimensions other than those in the direction of the molding pressure are similar to corresponding ones of the final molded member. Preferably, dimensions of the preliminary molded member in the direction of the molding pressure are set to be about 1.0 to about 2.0 times dimensions of the final molded member. When such a preliminary molded member is used, the mold density and the volume resistivity can be further improved.
According to the production method of the invention having the above-described molding means, the two-step molding is employed wherein a complex (bondcarbon compound) is previously cold-molded into a shape similar to the final molded shape by a pressure of a range of 2 to 10 MPa, and the preliminary molded member is placed in a mold and then molded into the final shape by applying a high molding pressure of a range of 10 to 100 MPa. Even when a complex (molding material) which is low in elongation and fluidity is used, therefore, the compound can surely extend to every corner of the mold so that, while suppressing molding unevenness, the mold density is increased and the complex can be charged more uniformly. As a result, it is possible to surely and easily obtain a uniform separator which exhibits low contact resistance and has good conductivity, and which is uniform and is correct also in shape.
As the thermosetting resin which is useful in the invention, phenol resin which is excellent in wettability with respect to graphite powder may be most preferably used. Alternatively, any other resin such as polycarbodiimide resin, epoxy resin, furfuryl alcohol resin, urea resin, melamine resin, unsaturated polyester resin, or alkyd resin may be used as far as the resin causes a thermosetting reaction when the resin is heated, and is stable against the operating temperature of the fuel cell and components of the supplied gasses.
As the graphite powder which is useful in the invention, powder of graphite of any kind, including natural graphite, artificial graphite, carbon black, kish graphite, and expanded graphite may be used. In consideration of conditions such as cost, the kind of graphite can be arbitrarily selected. In the case where expanded graphite is used, particularly, a layer structure is formed by expanding the volume of the graphite as a result of heating. When molding pressure is applied, layers can twine together to be firmly bonded to one another. Therefore, expanded graphite is effective in a complex in which the ratio of a thermosetting resin is to be reduced.
Other objects and effects of the invention will be clarified in embodiments which will be described below.